Responses of extracellular enzymes to simple and complex nutrient inputs

Abstract

Soil microbes produce extracellular enzymes that mineralize organic matter and release carbon and nutrients in forms that can be assimilated. Economic theories of microbial metabolism predict that enzyme production should increase when simple nutrients are scarce and complex nutrients are abundant; however, resource limitation could also constrain enzyme production. We tested these hypotheses by monitoring enzyme activities and nutrient pools in soil incubations with added simple and complex nutrient compounds. Over 28 days of incubation, we found that an enzyme's activity increased when its target nutrient was present in complex but not simple form, and carbon and nitrogen were available. β-Glucosidase and acid phosphatase activities also increased in treatments where only carbon and nitrogen were added. Glycine aminopeptidase and acid phosphatase activities declined in response to ammonium and phosphate additions, respectively. In some cases, mineralization responses paralleled changes in enzyme activity—for example, β-glucosidase activity increased and respiration was 5-fold greater in soil incubations with added cellulose, ammonium, and phosphate. However, a doubling of acid phosphatase activity in response to collagen addition was not associated with any changes in phosphorus mineralization. Our results indicate that microbes produce enzymes according to ‘economic rules’, but a substantial pool of mineral stabilized or constitutive enzymes mediates this response. Enzyme allocation patterns reflect microbial nutrient demands and may allow microbes to acquire limiting nutrients from complex substrates available in the soil.